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Digestion experiment. Meat has been placed in three test tubes containing hydrochloric acid (HCl, left), the protein-digesting enzyme pepsin (middle), and both HCl and pepsin (right). They were then heated to body temperature. The test tube with both pepsin and HCl (right) has entirely broken down the meat. This is because pepsin is secreted into the stomach in an inactive form known as pepsinogen. It is only when it comes into contact with HCl that it is converted into the active form that hydrolyses proteins into smaller peptones.